1. Field of the Invention
The present invention relates to a digital/analog conversion device converting digital data into an analog voltage as well as a display device indicating a gray level with the analog voltage generated by the digital/analog conversion device.
2. Description of the Background Art
A digital/analog conversion device which converts digital data into analog signals is commonly used. Such a digital/analog conversion device is used, for example, for generating an analog voltage so as to display a gray level (this voltage is hereinafter referred to as “gray-level voltage”) on a display device having, for each pixel, such a voltage-driven light-emitting device as a liquid-crystal display device or a current-driven light-emitting device of self-light-emitting type.
The above-described display device can display a gray level by setting the gray-level voltage to a level between the maximum brightness (white) and the minimum brightness (black) for each pixel. In other words, the gray-level voltage is set to one of 2n levels (n is a natural number) according to n-bit digital data and transmitted to each pixel.
One of generally known digital/analog conversion devices is constituted of a plurality of ladder-connected resistance elements (see for example “Exhaustive Guide to Analog IC with Illustrations” by Yoshio Shirato, Tokyo Denki University Press, November 1986, pp. 258-260). Such a ladder-type digital/analog conversion device, however, has a problem that the consumption current increases due to a constantly flowing DC current.
Then, a digital/analog conversion device is disclosed for example in Japanese Patent Laying-Open No. 2002-111499 (hereinafter referred to as “conventional art”) that has a charge pump circuit used to charge/discharge a capacitor element and thereby change an output voltage in a stepwise manner.
The digital/analog conversion device of the conventional art that uses the charge pump circuit can reduce its power consumption since no constant DC current is generated therein.
However, regarding the digital/analog conversion device of the conventional art that uses the charge pump circuit, a change in output voltage Vout between a pulse input and an immediately following pulse input varies depending on the level of the output voltage Vout. Specifically, as the output voltage Vout is higher, the change in output voltage per pulse input gradually approaches saturation.
Therefore, in order to set the output voltage Vout in such a manner that the output voltage changes at regular intervals, it would be necessary to control the number of clocks to be input to the charge pump circuit according to the level of the output voltage Vout, possibly resulting in a complicated circuit configuration. It is expected that this problem becomes noticeable when the digital/analog conversion device is employed for generating a gray-level voltage for a display device.